1. Field of the Invention
The present invention generally relates to a nitride semiconductor light emitting device, and, more particularly, to a nitride semiconductor light emitting device, which is reduced in crystal defects, and has improved luminance and reliability.
2. Description of the Related Art
Recently, a group III-V nitride semiconductor, such as a GaN semiconductor, has been in the spotlight as an essential material for light emitting devices, such as light emitting diodes (LEDs), laser diodes (LDs), and the like, due to its excellent physical and chemical properties. The III-V nitride semiconductor material comprises a GaN-based material having the formula InxAlyGa(1-x-y)N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1). In particular, LEDs or LDs of the nitride semiconductor material are mainly used for light emitting devices for emitting light in the blue or green wavelength band, and are used as a light source for many applications, such as video display boards, illuminating apparatuses, etc.
With an increasing demand of nitride semiconductor light emitting devices, the nitride semiconductor light emitting devices are required to have further improved luminance and reliability. However, the nitride semiconductor such as the GaN semiconductor is generally grown on a heterogeneous substrate such as a sapphire substrate, and thus suffers from crystal defects due to lattice mismatch between the semiconductor material and the heterogeneous substrate. The crystal defects provide an adverse influence on reliability (for example, tolerance to electrostatic discharge ESD) of the light emitting device, and deteriorate the luminance of the light emitting device via absorption of light. Although various attempts including a selective epitaxial growth method have been progressed to prevent the crystal defects, these attempts have problems of a complicated process such as deposition of SiO2 mask, and high manufacturing costs.
FIG. 1 is a cross-sectional view of a conventional nitride semiconductor light emitting device, particularly, showing a nitride semiconductor light emitting diode (LED). Referring to FIG. 1, the nitride semiconductor LED 10 comprises a buffer layer 13, an n-type GaN clad layer 14, an active layer 16, and a p-type GaN clad layer 18 sequentially stacked on a sapphire substrate 11. Some portions of the n-type GaN clad layer 14 and the p-type GaN clad layer 18 are exposed by mesa etching so as to allow an n-side electrode 24 to be formed on the exposed portion of the n-type GaN clad layer 14. A transparent electrode layer 20 formed from ITO and the like is formed along with a p-side electrode 22 on the p-type GaN clad layer 18. The buffer layer serves to relieve the lattice mismatch between the sapphire substrate and the GaN clad layer 14, and can be formed from AlN or GaN at low temperatures. Japanese Patent Laid-open Publication No. (Hei) 10-135514 discloses a nitride semiconductor light emitting device comprising an active layer having the multi quantum-well structure which comprises an undoped GaN barrier layer and an undoped InGaN well layer in order to improve efficiency of light emission.
Even with the buffer layer 13, however, the problems caused by the crystal defects within the light emitting device 10 cannot be satisfactorily solved. A significant density of crystal defects remains in the nitride semiconductor crystal, in particular, in the active layer. The crystal defects not only obstruct light emission from the active layer through absorption of light, but also reduce a reverse breakdown voltage or a tolerance voltage to reverse ESD. In this manner, the crystal defects deteriorate the luminance and reliability of the light emitting device.